Cube-on-edge oriented silicon-iron having improved magnetic properties and method for making same

ABSTRACT

Cube-on-edge oriented silicon-iron having improved magnetic properties and a glass free surface, and method for making it, wherein prior to the final anneal, the silicon-iron is coated with an annealing separator comprising a coarse, high purity alumina. The alumina separator may contain additives such as a grain growth inhibitor, a binder, and a compound which will aid in the removal of sulfur from the silicon-iron during the secondary grain growth stage of the final anneal but will not contaminate or form a film on the surface of the silicon steel.

United States Patent 1191 Jackson 1451 Jan. 15,1974

1 1 CUBE-ON-EDGE ORIENTED SILICON-IRON HAVING IMPROVED MAGNETIC PROPERTIES AND METHOD FOR MAKING SAME [75] Inventor: John M. Jackson, Middletown, Ohio [731 Assignee: Armco Steel Corporation,

Middletown, Ohio 122 Filed: Dec.2l, 1970 211 App]. 140.; 100,504

52 US. Cl 148/113, l48/31.5, 148/112 [51] 1111.0. 11011 1/04 581 Field of Search 148/112, 113, 31.5,

[56] References Cited UNITED STATES PATENTS 3,540,948 11/1970 Bcnford ct a1. 148/112 3,615,903 10/1971 Perry et a1. 148/122 3,073,722 1/1963 Hoehn ct al. 117/127 3,211,576 10/1965 Forslund ct 31.... 117/127 3,262,754 7/1966 Lindsay et al. 106/65 3,379,581 4/1968 Kohler et a1. 148/113 3,586,545 6/1971 Stanley 148/113 3,523,837 8/1970 Pavlik 148/113 3,333,991 8/1967 Kohler 148/113 X 3,132,056 5/1964 McQuade 148/113 X 3,152,930 10/1964 Foster 148/113 3,585,085 6/1971 Fritz 148/110 2,282,747 11/1966 Foster et al.. 148/113 3,333,992 8/1967 Kohler 1 148/113 3,333,993 8/1967 Kohler 148/113 Primary Examiner-L. Dewayne Rutledge Assistant ExaminerW. R. Satterfield AttorneyMelvi1le, Strasser, Foster & Hoffman [57] ABSTRACT 1 Claim, No Drawings CUBE-ON-EDGE ORIENTED SILICON-IRON HAVING IMPROVED MAGNETIC PROPERTIES AND METHOD FOR MAKING SAME BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to the production of cube-onedge oriented silicon-iron having improved magnetic properties through the use of an alumina annealing separator during the final anneal, and more particularly through the use of a coarse, high purity alumina.

2. Description of the Prior Art At the present time, cube-on-edge oriented siliconiron is in great demand in sheet gauge for magnetic uses such as laminated cores for transformers, and the like. Cube-on-edge oriented silicon-iron may be made by various routings. In general, however, the manufacturing process includes the basic steps of refining the base metal by known methods and forming the metal into an intermediate gauge product while hot. The intermediate gauge product can be made by continuous casting procedures, or by producing ingots and then hot rolling to intermediate gauge either in an uninterrupted procedure, or by producing slabs which are reheated and rolled on the continuous hot mill.

The intermediate gauge hot-reduced material either in sheet or strip form is preferably subjected to an annealing treatment prior to cold rolling. The material is then cold rolled to final gauge in one or more cold rolling treatments with an intermediate anneal or anneals if plural-stage cold rolling is practiced. The material is subjected to decarburization at some stage during the processing, and usually after cold rolling.

The silicon-iron is coated with an annealing separator and subjected to a final anneal having a primary grain growth stage and a secondary grain growth stage. In the primary grain growth stage, an inhibitor is present at the grain boundaries of the silicon-iron and tends to preclude the grain growth which would normally occur and produces a product in which the cube-on-edge nuclei occupy the lowest energy position. Thereafter, in the secondary grain growth stage, wherein the temperature reaches about 2,000 F. or above, the silicon-iron is substantially completely converted to a cube-onedge orientation through the grain boundary energy phenomenon.

It has long been known that alumina, per se, may be used as an annealing separator during the annealing of silicon-iron. It has additionally been known that certain types of alumina will not react with the silicon-iron surface to produce a glass film and may be removed after the annealing treatment by brushing or the like. Finally, it is known that alumina will tend to remove sub-surface silica, which forms in the silicon steel when subjected to high temperatures.

However, the use of alumina as an annealing separator for silicon-iron has been confined to the production of cube-on-face oriented silicon-iron. This is exemplified, for example, in U.S. Pat. No. 2,992,951; 3,152,9- 29 and 3,282,747. An alumina annealing separator is of particular advantage in the production of cube-onface oriented silicon'iron because this orientation is achieved through the surface energy phenomenon and the alumina annealing separator is porous and allows the annealing atmosphere to contact the surface of the silicon-iron. The surface energy phenomenon requires the surface of the silicon-iron to be free of glass, contaminants or the like.

With respect to the production of cube-on-edge oriented silicon-iron, a number of prior art patents (such as U.S. Pat. No. 3,333,992) have listed alumina as a possible annealing separator. Nevertheless, prior art workers have, in general, shunned its use because of its inferior retention upon the surfaces of the siliconiron stock prior to the final anneal and because it demonstrates no desulfurization effect during the secondary grain growth stage of the final anneal. In U.S. Pat. No. 2,385,322 it was taught that magnesia comprises an excellent annealing separator for the production of cube-on-edge material. During the final anneal, the magnesia forms a mill glass which is hard, tightly adherent and electrically insulative. In many applications, such an insulative coating is desired on the cubeon-edge material so that a product made of laminations of the material will have its laminations electrically insulated from each other. In U.S. Pat. No. 2,906,645 an improved magnesia coating is taught. The teachings of this patent have been followed almost exclusively by the industry since 1959.

Even in instances where a mill glass is not desired, such as in the production of cube-on-edge oriented silicon-iron sheet stock having good die life, prior art workers have felt constrained to use magnesia or some other glass-forming material with additives which will render the glass formed on the silicon-iron more readily removable. This is exemplified by U.S. Pat. No. 3,375,144.

The present invention is based upon the discovery that if coarse, high purity alumina is used as the annealing separator in the final anneal, the resulting product will not only be characterized by a better H=10 permeability, but also will be characterized by appreciably lower core loss values. After the final anneal, the alumina annealing separator particles may be readily removed. No glass is formed on the product during the final anneal and the product will serve well as a punching quality silicon-iron. If, on the other hand, an insulative coating is required on the final product, any of the well known insulative coatings may be applied thereto after the final anneal and the removal of the alumina annealing separator.

SUMMARY OF THE INVENTION In accordance with the present invention, silicon-iron is processed to produce a cube-on-edge oriented product by any suitable and well known routing though the hot and cold reduction steps and the decarburization step. Immediately prior to the final anneal, the siliconiron is coated with an alumina annealing separator. Thereafter, the silicon-iron is subjected to a final anneal including a primary grain growth stage and a secondary grain growth stage. The end product is glassfree and is characterized by superior permeability and appreciably lower core loss values.

The alumina, used as the annealing separator, must be relatively coarse, characterized by a high melting point and very pure. The alumina'anneal ing separator may contain one or more additives such as a binder, a primary grain growth inhibitor, such as sulfur, selenium or compounds thereof and a compound capable of removing sulfunfrom the silicon steel but which will not react with carbon or any other element in the silicon-iron to contaminate the silicon-iron.

As used herein and in the claims, the term siliconiron" is intended to designate a ferrous material containing at least about 2.0 percent and preferably from about 3.0 percent to 5.0 percent or more silicon, and from about 0.04 percent to about 0. l percent manganese. The carbon content in the melt should usually be from about 0.02 percent to about 0.05 percent, but the product (as indicated above) should be subjected to a decarburization treatment as part of the routing. The silicon-iron will contain up to about 0.006 percent aluminum and up to about 0.009 percent nitrogen. The initial sulfur content will usually be from about 0.01 percent to about 0.03 percent, but additional sulfur may be added during the primary grain growth stage of the final anneal. After the secondary grain growth stage of the final anneal, the sulfur content will be reduced from about 0.025 percent to not more than about 0.004 percent and preferably to about 0.001 percent. The balance of the alloy should be substantially all iron excepting for normal trace impurities incident to the mode of manufacture.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the production of cube-on-edge oriented siliconiron of the present invention, that portion of the routing including the hot and cold reduction steps and the decarburization step does not constitute a limitation on the present invention. The steps are well known in the art and may be performed generally as outlined above. For example, the teachings of U.S. Pat. Nos. 3,333,991; 3,333,992; and 3,333,993 may be followed.

Prior to the final anneal, the silicon-iron should be provided with an alumina separator (A1 0 Care should be taken to use an alumina which does not have a particle size so fine as to insufficiently space the silicon-iron sheets or coil convolutions during the final anneal and thus preclude contact thereof by the annealing atmosphere. On the other hand, if the alumina is of a particle size which is too coarse, the coiling thereof becomes difficult because the coil convolutions tend to slide with respect to each other, resulting in telescoping of the coil. Satisfactory results have been achieved using an alumina having a particle size distribution from about 100 mesh to about 400 mesh (Tyler screen).

It is important that the alumina used does not react with the steel to form a surface film thereon. Therefore, the alumina should have a purity of about 99 percent or more, with only trace amounts of soda, silica, iron oxide or the like.

The annealing separator of the present invention may be applied to the silicon-iron by any suitable and well known method of application of powder in dry or wet form. Thus, if applied in dry form, electrostatic deposition, dusting, rolling, etc. may be used. Application in slurry-form may be accomplished by spreading, doctoring, spraying or the like. Since alumina does not contain water of hydration, the water in a slurry can be removed in the lower temperature portion of the final anneal, or by a heat treatment prior to the final anneal.

It is within the scope of the invention to add to the alumina separator a binder to increase the retention thereof upon the surfaces of the silicon-iron stock prior to the final anneal. Exemplary organic binders are taught, for example, in U.S. Pat. 3,375,144, and include polyvinyl alcohol, wheat paste or urea formaldehyde in a water vehicle and latex-type adhesives in a benzene or naptha vehicle. Additionally, acids such as formic, oxalic, citric and carbolic acids may be used.

The alumina annealing separator may also contain a small quantity of a grain growth inhibitor such as sulfur, selenium or compounds thereof, as taught in the above mentioned U.S. Pat. Nos. 3,333,991; 3,333,992 and 3,333,993. The purpose for such an addition to the annealing separator is to provide an inhibitor which will diffuse into the grain boundaries of the silicon-iron. This will assure that a sufficient quantity of grain growth inhibitor exists at the grain boundaries to prevent the grain growth which would normally occur during the primary grain growth portion of the final anneal so that the silicon-iron may be substantially completely converted to a cube-on-edge orientation during the secondary grain growth portion of the final anneal.

Finally, it is within the scope of the present invention to incorporate in the annealing separator a small amount of a compound which will aid in the removal of sulfur from the silicon-iron during the secondary grain growth portion of the final anneal, but which will not react with the silicon-iron to form a glass or other contaminant on the surface thereof. Suitable sulfur getting compounds, and the required quantities for use in the final anneal are well known, per se, in the art.

After the alumina annealing separator has been provided on the silicon-iron, the silicon-iron may be subjected to the final anneal, as is known in the art. This anneal is conducted in a non-oxidizing atmosphere such as dry hydrogen, or, under vacuum conditions, and at a high temperature of 2,000" P. or above to develop the cube-on-edge grain orientation.

After the final anneal, the alumina annealing separator may readily be removed from the silicon-iron by brushing or scrubbing. The silicon-iron will then have a clean, uncoated surface.

When desired, an insulative coating may be applied to the silicon-iron at any time after the removal of the alumina. Any known insulative coating, either organic or inorganic, may be applied at this stage. For example, a phosphate coating may be applied such as that described in U.S. Pat. 2,501,846.

EXAMPLE Samples, in the form of Epstein strips, were secured from a coil of silicon-iron which had been processed in conventional manner including the steps of hot rolling, two cold rollings with an intervening anneal and strip decarburizing. The silicon-iron had the following initial composition:

Carbon 0.028 percent Manganese 0.087 percent Silicon 3.24 percent Sulfur 0.026 percent The final high temperature anneal was practiced on the Epstein strip samples in the laboratory at a temperature of 2,200 F. for 24 hours in a dry hydrogen atmosphere. Immediately prior to the final anneal one half of the Epstein strip samples were coated with a magnesia annealing separator and the other half of the samples were coated with 99.0 percent pure alumina hav- 1. Cube-on-edge oriented silicon-iron sheet stock ing a particle size distribution of from about 100 mesh containing at least 2 percent silicon and not more than to about 400 mesh (Tyler screen). about 0.001 percent sulfur, said stock having a clean,

After the high temperature ann al th amp r smooth metal surface free of mill glass and having an tested to determine thei C r 1088 an p rm i i y 5 improvement in core loss characteristics of at least 7 characteristics. The results are given in the following percent as measured at 17 kil i h a 60 cycle table:

COMPARISON OF MAGNESIA AND ALUMINA COATED SAMPLES Sample 7 Thickness P /60 P 17/60 p H-IO Magnesia Alumina Magnesia Alumina Magnesia Alumina Magnesia Alumina 5 .502 .495 .725 .673 i822 1838 .5 .50] .490 722 .667 I825 1838 .4 505 .495 .725 .67l i822 1838 4 .504 .493 .727 .666 i822 1840 NNNN From the above table it will be readily apparent that, current, said stock being produced by the steps of recontrary to prior art belief, alumina is not merely an fining the base metal and hot forming the base metal equivalent alternative to magnesia as an annealing p- 20 into an intermediate product, cold rolling the product arator. Not only do the alumina Coated Sampl s, when to final gauge, decarburizing the stock, and thereafter compared to the magnesia coat d Samp s, displ y coating the stock with alumina having a purity of at proved permeabilities, but also they display improved least 99 percent and having particle size distribution of core losses. The improvement in core loss values befrom about 100 to about 400 mesh (Tyler screen), and comes even more marked at higher inductions. thereafter subjecting said stock to a final high tempera- The embodiments of the invention in which an excluture a neal, sive property or privilege are claimed are defined as follows: 

